Particle therapy is a technique for treating solid tumors that is potentially more precise than x-ray radiation therapy. Charged particles enter the patient at relativistic speeds, depositing increasingly more dose as they come to rest inside the patient. X-ray photon beams deliver an exponentially decaying dose along the beam path, dosing healthy tissue before and after the tumor. When patient alignment is accurate, proton therapy causes less collateral damage overall, delivering less dose to proximal tissue and sparing distal tissue altogether. Today, patients are positioned by matching bony structures in daily x-ray images to planning CT volumes acquired days (or weeks) prior. Inter-fraction changes to soft tissue (weight loss, edema) are common and intra-fraction changes (digestion, respiration, etc.) are unavoidable. Range errors incorrectly dose healthy tissue and undertreat the tumor, limiting benefits of proton therapy. Therefore, the American Society for Therapeutic Radiation Oncology currently supports proton therapy for tumors near the base of the neck, spine, eye and in pediatric patients, and only in the context of clinical trials for most other tumor sites. Thermoacoustic range verification relies upon stress confinement and is suitable for compact synchrocyclotrons recently introduced by two manufacturers. My lab was the first to overlay thermoacoustic range estimates onto ultrasound images of the underlying morphology using a single transducer array to ensure inherent co-registration. The work was performed at national laboratories using low energy beams that generated high frequency thermoacoustic emissions which could be detected by ultrasound imaging arrays. The next step is developing a thermoacoustic range verification prototype for particle therapy systems that pulse delivery of high energy (200 MeV or more) particles. Range straggle smooths the Bragg curve and bandlimits thermoacoustic emissions below the sensitivity band of ultrasound imaging arrays. Therefore, low frequency receivers that can detect low-frequency (DC-100 kHz) thermoacoustic emissions will be added to higher frequency (1-15 MHz) biplane ultrasound imaging arrays. Phase I will be devoted to developing a prototype that overlays thermoacoustic range estimates into a 3D visualization of orthogonal biplane ultrasound images. Thermoacoustic range estimates will be updated at a rate of 10 Hz, assuming dose is delivered sufficiently quickly. If successful, deployment of a clinical prototype for low-risk studies to collect thermoacoustic data during the normal course of treatment will follow in Phase II.